Nitride gallium (GaN) is generally known as a compound semiconductor material suitable for blue light emitting devices or high-temperature electronic devices. Recently, the blue light emitting devices have been extensively used, so demands for GaN substrates have been increased. However, high-quality GaN substrates may not be easily manufactured, so that the manufacturing cost and the manufacturing time for the GaN substrates may be increased.
Different from silicon or sapphire, the GaN cannot be grown in the form of an ingot, so an epitaxial growth method is adopted to grow the GaN on a heterogeneous substrate, such as a SiC substrate or a sapphire substrate. Since there is difference in lattice constant and thermal expansion coefficient between the heterogeneous substrate and GaN crystal, dislocation density becomes high so that characteristics of devices employing the GaN substrate may be degraded and various problems may occur when manufacturing the devices.
In order to reduce the problems, the manufacturing process is complicated and the manufacturing time is increased. In the case of the ELO (epitaxial lateral overgrowth), which is extensively used to manufacture the high-quality GaN substrate, the stress caused by difference in lattice constant and thermal expansion coefficient between the substrate and GaN crystal is blocked by using a SiO2 mask having a stripe pattern. That is, according to the ELO scheme, after growing the GaN layer on the substrate, the substrate having the GaN layer is unloaded from a reactor and then the substrate is loaded into deposition equipment to deposit a SiO2 layer on the GaN layer. Then, the substrate having the SiO2 layer is unloaded from the deposition equipment and a SiO2 mask pattern is formed on the substrate through a photolithography process. Then, the substrate is again loaded into the reactor to complete the formation of the GaN layer (see Korean Patent Publication No. 455277). However, such an ELO scheme is very complicated, so that the process time is lengthened and reproducibility and the product yield are lowered.
Meanwhile, the light emitting device employing the compound semiconductor must have improved light emitting efficiency and reduced power consumption. That is, light emitted from an active layer of the light emitting device is guided toward a surface of the light emitting device as well as the substrate, so that the light is absorbed in the substrate, resulting in degradation of the light emitting efficiency. In order to solve this problem, a patterned sapphire substrate having a fine surface is employed to scatter the light guided toward the substrate in such a manner that the amount of light absorbed in the substrate can be reduced while increasing the amount of light guided toward the surface of the light emitting device. However, complicated processes and long process time are required in order to process the substrate having the fine surface.
Instead of the SiC substrate and the sapphire substrate, a low-priced silicon wafer having a large diameter can be used as a base substrate for the GaN growth. However, since there is greater difference in lattice constant and thermal expansion coefficient between the silicon substrate and the GaN crystal, the high-quality GaN substrate may not be obtained. In addition, the light emitted from the active layer of the light emitting device is absorbed in the silicon substrate due to the low band gap energy and opaque property of the silicon, so that the light emitting efficiency may be lowered.
As mentioned above, expensive and complicated processes, such as the photolithography process, are required to manufacture the high-quality compound semiconductor substrate having reduced crystal defect. In addition, although the expensive and complicated processes are performed to reduce the power consumption, the reproducibility and the product yield are still lowered.